Conventionally, with respect to caps which adhere to the mouth sections of metal bottles made of steel, aluminum alloy or the like, it is known that vent holes (also called knurl slits or vent slits) are formed in the vicinity of the upper end of the cap for purposes of releasing internal pressure and discharging gas from within the bottle body to the exterior when the cap is opened by rotational manipulation. For example, Patent Document 1 describes a cap including: a cap body in which multiple protrusions project from the inner surface of a skirt wall in the vicinity of a ceiling wall; and a liner which engages with the aforementioned protrusions at multiple sites at the rim, and which is arranged on the inner side of the ceiling wall in a non-adjoined state; wherein the aforementioned protrusions are formed by cutting incisions at the periphery of the cap body, and by pressing and bending the cut sections toward the inner side of the cap. With this cap, the incisions forming the aforementioned protrusions function as vent holes.
Patent Document 2 describes a cap that is screwed onto a mouth section of a bottle, including: a cap body which is provided with a top plate section, and a peripheral wall section that substantially hangs down from the rim of the pertinent top plate section; and a liner which is arranged so as to cover the inner surface of the top plate section on the inner side of the pertinent cap body; wherein liner supports which successively extend inward in the radial direction of the cap body from the rim of the top plate section are multiply formed at intervals in the circumferential direction at the top of the peripheral wall section, and the liner is arranged at the upper end of the pertinent liner supports in the axial direction. With this cap, slits are formed as vent holes at the bottom end of the recesses of knurls that are multiply formed in the circumferential direction of the peripheral wall section, and the recesses serve as the liner supports. In addition, with this cap, the slits and liner supports that constitute the vent holes are formed after prearrangement of the liner on the inner surface of the top plate section.
Thus, with the aforementioned conventional caps, the protruding portions of slits formed as vent holes are pressed inward to serve as protrusions or liner supports, and these serve as hooks which prevent displacement by engaging with the liner.